A large number of chemical and biological sensors are based on changes in the properties of a chemically sensitive material, such as changes in conductivity, surface charge or luminescence, that occur upon adsorption of analyte molecules. Analytes are, of course, the gas to be chemically analyzed. These changes can be monitored through physical methods and are related to the concentration of the analyte in the environment. In order to reduce the time associated with the diffusion of the analyte to the sensor site and to increase the amount of analyte seen by the sensor, sampling methods are used which force the air from the environment into direct contact with the sensitive polymer or other sensor material.
A typical present day configuration includes a material having physical properties that change when its surface is exposed to a gas containing certain chemical or biological species. These properties may be optical, electrical or mechanical, for example. A gas sampling system is used to bring fresh gas samples into contact with the surface of the material. Then, a read-out and signal processing system of electronics is used to convert the physical change to a useful output.
While these sensors have been demonstrated to work over a short time basis, they have been found to be adversely affected by long term drift that limits their practical use. The baseline drift frequently exceeds the minimum detectable signal by orders of magnitude, so that a sensitive technique is rendered essentially useless. It would be of great advantage to the art to greatly reduce the effects of baseline drift in gas sensing systems.
It would be another great advance in the art if a system would be developed to enable or significantly enhance the use of a wide variety of chemical and biological sensing techniques now not useful due to an inability to distinguish between the response to an analyte and the effect of baseline drift.
Other advantages will appear hereinafter.